The present invention relates to a method and to a system for converting a voice signal that reproduces a source speaker's voice into a voice signal that has acoustic characteristics resembling those of a target speaker's voice.
Sound reproduction is of primary importance in voice conversion applications such as voice services, oral man-machine dialogue and voice synthesis from text, and to obtain acceptable reproduction quality the acoustic parameters of the voice signals must be closely controlled.
The main acoustic or prosody parameters modified by conventional voice conversion methods are the parameters relating to the spectral envelope and, in the case of voiced sounds involving vibration of the vocal chords, the parameters relating to their periodic structure, i.e. their fundamental period, the reciprocal of which is called the fundamental frequency or pitch.
Conventional voice conversion methods are essentially based on modifications of the spectral envelope characteristics and on overall modifications of the pitch characteristics.
A recent study, published on the occasion of the EUROSPEECH 2003 conference under the title “A new method for pitch prediction from spectral envelope and its application in voice conversion” by Taoufik En-Najjary, Olivier Rosec, and Thierry Chonavel, foresees the possibility of refining the modification of the pitch characteristics by defining a function for predicting those characteristics as a function of spectral envelope characteristics.
Their approach therefore modifies the spectral envelope characteristics and modifies the pitch characteristics as a function of the spectral envelope characteristics.
However, that method has a serious drawback in that it makes modification of the pitch characteristics dependent on modification of the spectral envelope characteristics. An error in spectral envelope conversion therefore inevitably impacts on pitch prediction.
Moreover, the use of a method of the above kind requires two major calculation steps, namely modifying the spectral envelope characteristics and predicting the pitch, thereby doubling the complexity of the system as a whole.